The use of light for measuring certain physical and chemical characteristics has been known in laboratories for some years. For example, both qualitative and quantitative analyses are frequently made using spectroscopic techniques. The advent of both lasers and optical fibers have greatly increased the activity in this field. In particular, optical fibers have allowed the locating of sensitive and expensive equipment remote from harsh reactor environments, thus making light analysis techniques suitable for application to commercial processes.
One analytical technique that could be useful for commercial applications is Raman spectroscopy. When light of a single wavelength interacts with a molecule, the light scattered by the molecule contains small amounts of light with wavelengths different from that of the incident light. This is known as the Raman effect. The wavelengths present in the scattered light are characteristic of the structure of the molecule, and the intensity of this light is dependent on the concentration of these molecules. Thus, the identities and concentrations of various molecules in a substance can be determined by illuminating the substance with light of a single wavelength and then measuring the individual wavelengths, and their intensities, in the scattered light. The details of Raman spectroscopy are discussed in U.S. Pat. No. 3,556,659 wherein further references are also cited for the theory of the Raman effect.
A major difficulty associated with Raman spectroscopy is the low intensity of the scattered light compared to the exciting light. Elaborate spectrometers, having high light gathering power and dispersion, high stray light rejection, and sensitive detectors, are required to isolate and measure the low intensity Raman scattered light. These instruments are costly and sensitive, and thus are not well suited for use in commercial manufacturing or processing facilities. As a result, they have rarely been used outside of laboratory environments. A simplified fiber-optic probe that could be located at a point remote from its light source and from its spectrometer could make Raman spectroscopy available for analyzing commercial processes.
For light measurements, a sensing probe is normally designed to maximize the overlap between the are illuminated by a fiber transmitting light into a sample and the area viewed by a fiber collecting light from the sample. Ideally, then, a single fiber should be used for both transmitting and collecting light as discussed by Hirschfeld in his article "Remote Fiber Fluorimetric Analysis," Defense Programs, p. 17. This is not feasible, however, in Raman spectroscopy due to the low intensity of the Raman scattered light. The light being transmitted down the fiber excites the molecules of the fiber itself and thus generates Raman scattering within the fiber which interferes with Raman scattered light collected by the fiber from the sample. A multifiber probe, therefore, with the fibers performing independent functions of transmitting and collecting, must be used.
A probe having two fibers is described in Japanese Pat. No. 55-112549. That reference, however, teaches that lenses are necessary at the fiber ends for focusing the laser (exciting) light and for collecting the Raman (scattered) light. The proper alignment of these focusing lenses is necessarily critical to achieve a common image point, but such alignment is difficult to achieve due to the very small diameters (typically less than 1000 microns) of the fibers. The focusing lenses thus greatly add to the cost and complexity of the probe and thereby render it less attractive for commercial processes.
The present invention offers a significant improvement in that it describes a relatively simple probe, useful for Raman analysis, that can be easily modified for many diverse applications.